1. Field of the Invention
The present invention relates to methods for degasifying subterranean coal-containing formations employing hydraulic fracturing techniques.
2. Brief Description of the Prior Art
It has long been known that many subterranean coal-containing formations contain significant quantities of methane gas and other gaseous materials. The porous fractured structure of coal, which contains an extensive network of ultrafine capillaries that pass in all directions through the coal, facilitates the movement of the gases through the coal. Because of the hazardous nature of methane gas and its ability to move through the coal, it has not been possible for miners to produce coal from formations containing large quantities of methane gas.
Recently, it has been found that by drilling boreholes into subterranean coal-containing formations that the methane gas and other gases can be drained from the formation and the gas can be recovered at the earth's surface. In some instances, sufficient quantities of gas are present to make it economically desirable to recover the gas for introduction into a commerical gas pipeline for sale, even though the coal deposit may not be of a quality sufficient to justify mining. Various attempts have been made to facilitate the flow of the gas from the subterranean formations, however, these methods have been hindered by the brittle, friable nature of coal.
One method has employed the creation of artificial fractures in the coal-containing formation in accordance with conventional fracturing treatments to facilitate hydrocarbon production from subterranean oil and gas formations. The artificial fracturing process causes the formation of many minute fractures in the coal in the face of the principal fractures. Because of the brittle nature of the coal, these fractures cause small particles to separate from the face of the principal fracture. Production of methane or other gases from the coal also causes spalling from the face of the fracture in the coal seam as the gas passes through the face. The fine particles of coal and other materials created by the fracturing process and by the passage of gases through the coal lodge in and seal off the natural and artificially created fractures in the coal seam and thereby substantially hinder the flow of gas through the coal.
It would be desirable to provide a method by which a subterranean coal-containing formation can be fractured and the fracture can be maintained substantially unblocked by spalled coal particles to permit the drainage of methane gas from the formation and subsequent mining of the coal.